how would i fix this error:

"Exception in thread "main" java.lang.NullPointerException: Cannot read the array length because "tictactoegame.TicTacToeGame.gameboard" is null

at TicTacToeGame$/$tictactoegame$.$TicTacToeGame.displayBoard$($TicTacToeGame$.$java:$18)$

at TicTacToeGame$/$tictactoegame$.$TicTacToeGame.main$($TicTacToeGame$.$java:$49)"$

package tictactoegame;

public class TicTacToeGame $\{$

static int$[][]$ gameboard;

static final int EMPTY $=0$;

static final int NOUGHT $=-1$;

static final int CROSS $=1$;

$/*$ Set a square on the board must be empty $*/$

public static void set$($int val, int row, int col$)$ throws IllegalArgumentException $\{$

if $($gameboard$[$row$][$col$]==$ EMPTY$)$

gameboard$[$row$][$col$]=$ val;

else throw new IllegalArgumentException$("$Player already there!"$)$;

$\}$

$/*$ display the board $*/$

public static void displayBoard$()\{$

for$($int r$=0$; r